Pedro Aguiar (Universidade do Porto, Faculdade de Ciências, Portugal); Clara Lázaro (Universidade do Porto, Faculdade de Ciências, Portugal); M. Joana Fernandes (Universidade do Porto, Faculdade de Ciências, Portugal)

Launched on 21 November 2020 and 17 November 2025, respectively, Sentinel-6 Michael Freilich (S6MF) and Sentinel-6B (S6B) satellites carry an Advanced Microwave Radiometer for Climate (AMR-C), which, in addition to the standard low frequency channels, includes a High-Resolution Microwave Radiometer (HRMR). This subsystem allows higher spatial resolution for enhanced Wet Tropospheric Correction (WTC) measurements in coastal zones. The most recent processing baseline G01 products provide two different WTC fields: AMR WTC, computed from AMR measurements alone, and RAD WTC, computed from the combination of AMR and HRMR. The aim of this study is to evaluate G01 high-resolution WTC over the global coastal regions, in particular to quantify the performance of the RAD WTC when compared with the AMR WTC. This research is carried out in three main steps: i) comparison of both S6MF WTC with ERA5 and the WTC derived from global navigation satellite systems (GNSS), during the first five years of the satellite (2021–2025); ii) the same comparisons for S6B during the first cycles; and iii) comparison between S6MF and S6B during the tandem phase.
For the evaluation of S6MF WTCs, results show that, on average, at distances 0-5 km from the coast, RAD WTC is available for only 13% of S6MF points. The inter-comparison between both corrections reveals the largest differences for distances to land between 5 and 10 km, with a mean RMS of 0.9 cm for the whole period, while for distances in the range 20-80 km the mean RMS decreases to 0.3 cm. Comparisons with ERA5 and GNSS reveal that the new RAD WTC is better than the AMR WTC for distances to coast in the range of 5-20 km, with an RMS reduction up to 0.2 cm. Additionally, a substantial number of S6MF points with valid RAD values were recovered for cycles 5 to 8, after the transition from processing baseline F08 to G01.
For S6B, the same analysis reveals a similar availability of RAD WTC in coastal regions. The largest differences between AMR and RAD WTC, with a mean RMS of 2.8 cm for the whole period, are also observed for distances to land between 5 and 10 km, decreasing to 1.3 cm at 20-80 km. As mentioned above, the corresponding statistics for S6MF are 0.9 and 0.3 cm, respectively. Moreover, coastal comparisons with ERA5 and GNSS show larger differences for RAD than for AMR WTC. For distances of 5-20 km from coast, where the S6MF RAD WTC outperforms the corresponding AMR WTC, the RMS of the differences for S6B are 2.0 cm (ERA5-AMR) and 3.1 cm (ERA5-RAD). For the same distance range, the RMS of the differences GNSS-AMR and GNSS-RAD are 1.6 and 2.4 cm, respectively. Over the open ocean, the comparison ERA5-S6B reveals similar statistics for both S6B WTC, with an RMS around 1.0 cm.
During the S6MF-S6B tandem phase, over the open ocean, S6B measures wetter than S6MF by 0.1 cm (AMR) and 0.2 cm (RAD), with an RMS of the differences S6MF-S6B of 0.4 cm for both AMR and RAD. For the RAD WTC, the RMS of S6MF-S6B differences exceeds 1 cm for distances less than 80 km from the coast. For the AMR WTC, the corresponding RMS exceeds 1 cm only for distances less than 20 km from the coast. This indicates that the two satellites show better agreement for the AMR WTC than for the RAD WTC.
Considering the first cycles of S6B, this study finds opposite results for S6MF and S6B. The high-resolution radiometer on S6MF improves WTC retrieval at 5-20 km from the coast, allowing higher recovery of accurate sea level measurements in these regions. However, for the current S6B products, the same improvement is not observed, with AMR WTC closer to ERA5 and GNSS than RAD WTC.